This invention relates to an incremental length or angle measuring instrument of the type having a measuring scale which defines a grid, a plurality of reference marks fixedly positioned with respect to this scale, and means for sensing the reference marks and generating a reference signal in response thereto.
In such measuring instruments, electronic signals generated at the reference marks are used in various manners. For example, such electronic signals can be used to set the counter display to zero or to load a predetermined starting value into a counter at a certain position along the scale. Furthermore, such reference marks can be used for correction of the counter and the control of interference pulses.
In one type of known prior art measuring instrument, the number and position of the reference marks are determined in the production of the measuring scale, in accordance with the needs of the user. In such measuring instruments it is not possible during the later use of the measuring instrument to change the number or position of the reference marks, or of the electronic signals derived from the reference marks.
U.S. Pat. No. 4,101,764 discloses a measuring instrument to overcome these disadvantages. As described in this patent, a measuring scale can be provided with a plurality of reference marks that are formed at the time the scale is manufactured and are fixed at predetermined points along the scale. In addition, the disclosed measuring device includes at least one switching means on the scale itself or in its immediate vicinity. This switching means is used to select one or more reference marks such that it is only the selected reference marks which go into effect, and unselected marks have no effect on the operation of the measuring device. This patent discloses a switching means which includes a plurality of magnets, one of which is associated with each reference mark that is to go into effect. A scanning unit is provided to scan the scale, and this scanning unit includes an electrical reed switch which is responsive to the magnets. The electrical output of the reed switch together with the electrical output of the scanning unit are both applied to an electronic component which generates a control pulse at its output only if an electrical signal is simultaneously present on the output of the scanning unit and on the output of the reed switch.
This prior art approach does act to enable only a selected subset of the total number of reference marks. However, experience has shown that electrical switches such as reed switches tend to chatter or bounce during the switching process, and such bounce can lead in turn to interference impulses and thereby to the false generation of control signals in the measuring system. Furthermore, the magnets utilized in this prior art approach to designate the selected reference marks can under unfavorable conditions attract debris such as metal shavings and the like. This debris can also cause disturbances of the measuring process and can adversely effect its accuracy.